1. Field of Invention
This invention relates generally to devices with rectifying properties that normally work under reverse biasing conditions.
2. Description of Related Art
Devices with rectifying properties, that may normally work under reverse biasing conditions, require the structural ability to include a bias voltage. Such devices may be multilayered structures with positively-doped, or P, layers, and negatively-doped, or N, layers. In some instances, one or several intrinsic, or I, layers, are also part of the device. Devices with rectifying properties and that normally work under reverse bias conditions may have, for instance, PN, NP, NIP or Schottky-junction structures. The related art discussed herein is specific to PIN photosensors for clarification purposes only, but the principles underlined and discussed can also be applied to other structural devices with rectifying properties that normally work under reverse biasing conditions.
High fill-factor structures are structures having a continuous top portion, and a pixelated bottom portion that defines a plurality of pixels or other repeated structures which are often arranged in an array or the like. The continuous top portion is often a positively doped layer or a transparent, electrically conductive layer. The pixelated bottom portion is often formed by negatively doped material and a metal contact layer. Such high fill-factor devices have been widely used in CMOS imagers with, for instance, positive-intrinsic-negative (PIN) photosensors and in flat panel images. One of the differences between a high fill-factor sensor array and a conventional sensor array is that, in a conventional sensor array, the top portion and the bottom portions are both pixelated. In contrast, in a high fill-factor structure, the top portion is continuous.
For example, PIN photosensors generally include a positively-doped layer, or p-type layer, an undoped or intrinsic layer, or I-layer and a negatively-doped layer, or n-type layer. In this configuration, the I-layer is excited by photons when the photosensor is exposed to light. When irradiated by photons, the I-layer generates electron-hole pairs that separate under the action of the electric field generated by the built-in potential and the supplied reverse bias, and drift through the I-layer. The electrons drift to the N-layer and the holes drift to the P-layer. When the electrons reach the N-layer through drifting, they travel through the N-layer and reach one or more signal electrodes. The signal electrodes then transmit those electrons to one or more downstream circuits.
When these PIN photosensors are arranged in an array, each individual photosensor generates an electrical current proportional to the amount of electrons that drift to the portion of the N-layer that is associated with that photosensor structure. Since the amount of electrons drifting out of the I-layer depends upon the light intensity that irradiates the I-layer, the amount of electrons that reach the N-layer and travel to the signal electrodes also depends on the intensity of the light that irradiates the portion of the I-layer that is associated with that particular semiconductor structure. In other words, the amount of charge that is generated by each individual photosensor depends directly on the intensity of the light irradiating the portion of the I-layer that is associated with that photosensor. Accordingly, different photosensors generate different charge levels in the same photosensor array as different portions of the I-layer are irradiated by different amounts of light.
For the electrons to drift through the I-layer and into the N-layer, a voltage bias should be applied to the photosensor structure. The most common technique used to apply this bias voltage to the photosensor structure is to add an additional transparent conductive electrode in the form of a layer on top of the P-layer, and to connect that transparent conductive layer to a voltage source.
The P-layer is thus generally covered by a transparent, electrically conductive layer. It should be understood that the transparent, electrically conductive layer needs to be transparent at the wavelength range of the electro-magnetic radiation that the photosensor is designed to sense. In general, this transparent, electrically conductive layer has no role in transmitting the light intensity signal and should not interfere with the light intensity signal by altering, for instance, the intensity or the wavelength of the light intensity signal. It should further be understood that the transparent, electrically conductive layer should be applied over the entire surface of the photosensor to provide a uniform distribution of bias voltage over the entire photosensor array. A uniform distribution of bias voltage ensures that the relative number of electrons that drift through the I-layer and to the N-layer is proportional only to the intensity of light that is being irradiated on the I-layer, i.e., that it is not due to local differences in bias voltage across the P-layer or across the I-layer.
U.S. Pat. No. 6,018,187 describes a PIN layer in which a transparent conductive layer, formed over the P-layer, is electrically connected to a bias voltage source.